Current and projected experiments using rodents to model the aging processes and age-related diseases in humans require animals of defined genotype and controlled environmental and health status. Only with the meticulous, long-term control of genetic and environmental variables is it possible to maintain relevant animal models that may be used to study many ofthe biological and behavioral processes in aging. Therefore, a continuous supply ofgenetically defined, well characterized inbred and hybrid laboratory animals is essential for a program of research on aging. The purpose of this project is to develop, maintain and distribute a standing colony of aged mice of NIA-specified genotypes for use by investigators in studies of aging.